1. Field of the Invention
The present invention relates to a taking lens system, and more particularly to a compact taking lens system suitable for use in a digital input device (such as a digital still camera or digital video camera) that takes in an image of a subject by means of a solid-state image sensor.
2. Description of the Prior Art
In recent years, as personal computers and the like become more and more popular, digital still cameras and digital video cameras (hereinafter collectively referred to as “digital cameras”), which permit easy entry of image data into digital equipment, have been becoming more and more popular for personal use among general users. Such digital cameras are expected to become even more widespread in the future.
Solid-state image sensors, such as CCDs (charge-coupled devices), used in digital cameras have been made increasingly small, and accordingly further miniaturization is sought in digital cameras themselves. As a result, miniaturization is eagerly sought also in taking lens systems, which occupy the largest volumes in digital input devices. The easiest way to make a taking lens system smaller is to make a solid-state image sensor smaller. However, this involves making photosensitive elements smaller, and thus makes the solid-state image sensor more difficult to fabricate, and in addition requires higher performance in the taking lens system.
On the other hand, making a taking lens system smaller while keeping the size of a solid-state image sensor unchanged inevitably brings the exit pupil position closer to the image plane. When the exit pupil position is brought closer to the image plane, the off-axial rays exiting from the taking lens system are obliquely incident on the image plane. This makes it impossible to make the most of the light-condensing ability of the microlenses provided in front of the solid-state image sensor, with the result that the obtained image shows extremely uneven brightness between in a central portion and in a peripheral portion thereof. This can be avoided by bringing the exit pupil position of the taking lens system farther away from the image plane, but this inevitably makes the taking lens system as a whole unduly large.
Moreover, in keen competition for lower prices in recent years, lower costs are increasingly eagerly sought in taking lens systems. To meet these requirements, Japanese Patent Application Laid-Open No. H6-88939 proposes a taking lens system composed of one lens element, and Japanese Patent Application Laid-Open No. H1-245211 proposes a taking lens system composed of two lens elements. Moreover, Japanese Patent Application Laid-Open No. 2001-183578 proposes a taking lens system composed of two lens elements for use with a solid-state image sensor. Furthermore, Japanese Patent Applications Laid-Open Nos. 2001-272598, 2002-698889, and H11-52227 propose taking lens systems composed of three lens elements.
These conventional taking lens systems, however, have the following disadvantages. The taking lens system composed of one lens element disclosed in Japanese Patent Application Laid-Open No. H6-88939 has the exit pupil position located far beyond the total length of the taking lens system, but suffers from poor image plane quality, and thus cannot maintain high imaging performance. The taking lens system composed of two lens elements disclosed in Japanese Patent Application Laid-Open No. H1-245211 offers good image plane quality, and thus maintains high imaging performance. However, here, the exit pupil position is located too close, namely at about 6/10 of the total length of the taking lens system. The taking lens system disclosed in Japanese Patent Application Laid-Open No. 2001-183578, in which the first lens element has a negative optical power and the second lens element has a positive optical power, has a long total length and is not compact.
The taking lens system composed of three lens elements disclosed in Japanese Patent Application Laid-Open No. 2001-272598, of which the total length is about three times its focal length, is not compact. The taking lens system disclosed in Japanese Patent Application Laid-Open No. 2002-698889, of which the total length is about twice its focal length, is compact, but uses two glass lens elements, which make it impossible to produce this taking lens system at low costs. The taking lens system disclosed in Japanese Patent Application Laid-Open No. H11-52227, of which the total length is about 2.5 times its focal length, is not compact. Moreover, here, the angle of view is about 40 degrees, which is too narrow to make this taking lens system usable.